1. Field
The invention relates to an RF device with a transmit/receive switch circuit.
2. Description of the Related Art
U.S. Pat. No. 5,689,818 discloses a transmit/receive switch circuit, i.e. a circuit that is able both to transmit signals to a terminal and receive signals from that terminal called a transmit/receive terminal. This circuit contains an RF transmit signal input; an RF receive signal output and the RF transmit/receive terminal. A transmit section has a first switching transistor between the RF transmit signal input and ground and a first pass switch coupled between the RF transmit signal input and the RF transmit/receive terminal. A receive section has a second pass switch coupled between the RF transmit/receive terminal and the RF receive signal output and a second short switch (switching transistor) between the RF receive signal output and ground. A control circuit controls the first and second short switch and the first and second pass switch.
In the transmit mode the first short switch is open (non-conductive) and the first pass switch is closed (conductive). In this mode the second short switch is closed and the second pass switch is open. In a receive mode the second pass switch is closed and the second short switch is open. In this mode the first short switch is closed and the first pass switch is open. U.S. Pat. No. 5,689,818 uses transistors (FETs) as switches. The control circuit opens and closes these switches by applying control voltages to the control electrodes (gates) of the switches.
U.S. Pat. No. 5,689,818 addresses the problem that the power supply voltage may be insufficient to provide the required control voltages when the power supply voltage is relatively low and the RF voltages are relatively high. U.S. Pat. No. 5,689,818 solves this problem by means of rectifier circuits, each comprising a diode and a capacitor in series. The rectifiers of U.S. Pat. No. 5,689,818 are only used in the transmit mode. A first rectifier circuit is coupled from the RF transmit signal input to the control electrode of the first short switch. This first rectifier circuit helps to lower the control voltage of the first short circuit adaptively with increasing transmit power. In this way the first short switch will not become conductive at high RF transmit power. A second rectifier circuit is coupled from the RF transmit/receive terminal to the control electrode of the second pass switch. This second rectifier circuit helps to lower the control voltage of the second pass circuit adaptively with increasing transmit power. In this way the second pass switch will not become conductive at high RF transmit power.
In the receive mode overload problems may arise when the received signal from the transmit/receive switch circuit is too large for the receiver. It is known to solve such problems by the use of a limiter between the antenna and the receiver. Typically, a limiter may contain a series connection of a limiter diode and a capacitor coupled between said connection and ground.